1. Field of Invention
The present invention relates to an improvement for axial flow fan motors which are provided in various O.A. equipments and so on.
2. Background of Related Art
Devices, such as various O.A. equipments, in which a large number of electronic components are accommodated within casings, may damage the electronic components due to the accumulated heat generated by the electronic components. An axial flow fan motor is attached to a ventilating hole provided through a side wall of the casing to release the internal heat of the casing externally.
An example of related axial flow fan motors will be described with reference to FIG. 3. A casing 1 has an inclined internal periphery that is formed as a venturi portion 1a. A cylindrical portion 1b is provided inside this casing 1. The outer races of two bearings 2 and 3 are supported by this cylindrical portion 1b so that these bearings 2 and 3 are spaced from each other. A shaft 5, provided at its one end with a washer 4, is fittingly supported by the inner races of the bearings 2 and 3. A central portion 6a of an impeller 6 is mounted onto this shaft 5. The impeller 6 is made of synthetic resin, and has a shape in section. A fan 6c is provided on an outer peripheral portion 6b of the impeller 6. A spring 7 is interposed between the central portion 6a of the impeller 6 and the bearing 2 to urge the impeller 6 upwardly in FIG. 3.
Outside the cylindrical portion 1b of the casing 1 is a PC board 8 on which a drive circuit and component parts therefor are mounted is provided. A stator assembly 10 is provided above the PC board 8, and the PC board 8 is fixed to the stator assembly 10 through a pin 9. The stator assembly 10 includes a stator yoke 10a and a coil 10b. A lead wire 11 is provided so as to supply electric current from a power source circuit (not shown) with the coil 10b through the drive circuit.
A motor yoke 12 is molded onto the outer peripheral portion 6c of the impeller 6. A magnet 13 is attached to this motor yoke 12 so as to be confronted with the stator assembly 10. As shown in FIG. 3, the motor yoke 12 has an L-shape, and a portion of the impeller 6, which corresponds in location to a bent portion of the motor yoke 12, is provided with a small hole 6d. This small hole 6d is formed as a consequence of the removal of a penetrated support rod which has been used to support the motor yoke 12 during molding. After the completion, the motor yoke 12 can be viewed from the outside through this small hole 6d.
The axial flow fan motor thus constructed operates as follows: That is, when the electric current is applied to the drive circuit incorporated in the PC board 8 through the lead wire 11 from the power source circuit (not shown), the drive circuit supplies the electric current to the coil of the stator assembly 10 to produce magnetic effect in cooperation with the motor yoke 12 and the magnet 13. Thus, the shaft 5 and the impeller 6 are rotated together. This rotation generates air flow between the fan 6c of the impeller 6 and the venturi portion 1a so that the air flow is directed upwardly in the drawing. The O.A. equipment or the like to which this axial flow fan motor is attached can be ventilated in this manner.
However, it was found out through an electrostatic test that the anti-electrostatic reliability can not be ensured between the impeller and the lead wire during the operation in this manner. That is, since a distance between the motor yoke molded on the outer peripheral portion of the impeller and the PC board is small particularly in a case of a miniature motor, the rotation of the impeller induces an electrostatic current between the motor yoke and the PC board. This results in the lowered anti-electrostatic reliability found through the electrostatic test between the impeller and the lead wire.